Treatment of phospho gypsum

ABSTRACT

A process is provided for recovering phosphoric acid from phospho gypsum produced as a by-product in a phosphoric acid production process and having water soluble P 2 O 5  bound in the crystal lattice thereof. The process includes a) forming a suspension of phospho gypsum in an aqueous medium; b) subjecting the suspension to ultrasonic waves under conditions suitable to shatter the phospho gypsum crystal lattice releasing the bound water soluble P 2 O 5  into the aqueous medium to produce phosphoric acid and a depleted phospho gypsum; c) separating the phosphoric acid from the depleted phospho gypsum and recovering the separated phosphoric acid; and d) optionally further treating the depleted phospho gypsum in a step to further reduce the level of any remaining water soluble P 2 O 5  present in the depleted phospho gypsum. The level of the water soluble P 2 O 5  in the depleted phospho gypsum is reduced to about 0.01% by weight.

BACKGROUND OF THE INVENTION

This invention relates to a process for treating phospho gypsum.

The manufacture of phosphoric acid, according to the wet process,consists in essence of reacting calcium phosphate rock with sulfuricacid. The calcium oxide in the rock reacts with the sulfuric acid toproduce calcium sulphate di-hydrate, also known as gypsum. This calciumsulphate crystallises out in the phosphoric acid medium in the reactor.Since this gypsum is a by-product of the phosphoric acid productionprocess it is commonly known as phospho gypsum.

The phospho gypsum is filtered and rinsed with water to recover as muchphosphoric acid as is economically justified. However, the phosphogypsum typically contains water soluble P₂O₅ bound in the crystallattice which is not recovered as phosphoric acid during the rinsingstage. As a result, phosphoric acid production plants lose substantialamounts of phosphoric acid, expressed as P₂O₅ bound in the gypsumcrystal lattice.

In the manufacture of Portland cement, gypsum is interground withclinker in order to regulate the setting and hardening process of thecement after addition of water. When phospho gypsum is used for thispurpose it is found that the setting times are extended to such a degreeas to interfere with normal operations on a construction site. It hasbeen found that these extended setting times are caused by the watersoluble P₂O₅ in the gypsum.

Whilst it is possible to convert the water soluble P₂O₅ adhering to theoutside of the gypsum particles to insoluble phosphates, by washing witha calcium hydroxide solution at ambient or elevated temperatures, theP₂O₅ in the crystal lattice is not accessible to the hydroxide underthese conditions. Hence phospho gypsum treated in this manner gives riseto variable setting times when used in cement production.

Several processes exist to free the P₂O₅ in the crystals. Some consistin essence of drying the gypsum and heating same to high temperatures inorder to break up the crystal structure thus releasing the P₂O₅.Limestone, having been added, dissociates at these high temperaturesforming calcium oxide which then reacts with the P₂O₅ to form waterinsoluble calcium phosphates. The product, which consists of calciumsulphate anhydrite and calcium phosphate, is then cooled and sprayedwith water to convert the anhydrite back to calcium sulphate hydrates.It should be noted that all P₂O₅ in the crystals is converted toinsoluble phosphates and is thus lost to the phosphoric acid productionprocess. It is clear that this process, and some variations of same, isenergy intensive because of the drying and subsequent dehydration of thegypsum and the heat required to dissociate the limestone. Capital outlayis also high. Hence this process is economically not attractive.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a process forrecovering phosphoric acid from phospho gypsum produced as a by-productin a conventional phosphoric acid production plant and having watersoluble P₂O₅ bound in the crystal lattice thereof, the processcomprising:

a) forming a suspension of phospho gypsum in an aqueous medium;

b) subjecting the suspension to ultrasonic waves under conditionssuitable to shatter the phospho gypsum crystal lattice releasing thebound water soluble P₂O₅ into the aqueous medium; and

c) recovering the phosphoric acid so released.

The depleted phospho gypsum is preferably treated in a further step toreduce any remaining water soluble P₂O₅ to a level which allows thegypsum to be added to a cementitious material for use as a settingregulator.

The level of water soluble P₂O₅ in the gypsum is preferably reduced tobelow 0.06% by weight, in particular to about 0.01% by weight.

The suspension is preferably subjected to ultrasonic waves of less than2 MHz, typically 50 to 500 Hz or 16 kHz to 2 MHz.

During the subsequent treatment of the phospho gypsum, typicallysonication of the depleted phospho gypsum in an aqueous medium, aneutralising agent may be added to the aqueous medium to convert thewater soluble P₂O₅ into an insoluble phosphate.

According to a further aspect of the invention, there is provided acement comprising phospho gypsum treated in accordance with theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The crux of the invention is a process for dislodging P₂O₅ bound in thecrystal structure or lattice of phospho gypsum, recovering the dislodgedP₂O₅, and the further treatment of the depleted gypsum to render itsuitable for use in either the cement industry or the gypsum productsindustry or both.

It has been reported in the literature that ultrasonic waves of suitablewavelength and intensity cause cavitation in a liquid medium. The smallbubbles collapse giving rise to localised “hotspots” having very hightemperatures and pressures, in addition to shockwaves. It has now beenfound that this phenomenon can be used for the shattering of a phosphogypsum crystal lattice thus releasing P₂O₅ bound therein.

In general the process consists of forming a suspension or slurry ofphospho gypsum derived as a by-product in a conventional phosphoric acidproduction plant in an aqueaus medium, typically water, and subjectingthe suspension to ultrasonic waves of a suitable wavelength. Thisprocess leads to the disruption or shattering of the phospho gypsumcrystal lattice thus releasing the water soluble P₂O₅ bound in thecrystal lattice to form phosphoric acid. Typically a wavelength of lessthan 2 MHz will be used to break up the crystal lattice. Examples ofsuitable wavelength ranges are 50-500 Hz and 16 kHz-2 MHz, based oneconomic and end use requirements.

Subsequent filtration and drying at low temperatures produces a gypsumproduct eminently suitable for the cement industry and for the generalgypsum products industry whilst providing for the economic recovery ofphosphoric acid.

Whilst laboratory tests have shown that a wide range of frequencies canbe used, a sonicator suitable for industrial purposes has been found tobe the patented Nearfield Acoustic Processor (NAP) manufactured by theLewis Corporation of the USA. This machine has two opposing diaphragmplates vibrating at two different frequencies, namely 16 kHz and 20 kHz.These plates form the two active walls of the reaction chamber throughwhich the slurry is pumped. The intensity of the vibrations can beadjusted as required, whilst the gap between the plates can be variedbetween 0.12 and 25.4 mm.

EXAMPLE

Phospho gypsum slurry was passed through a 10 mm gap between thevibrating reactor surfaces in a 2 kW NAP. The power was set at maximum.

Table 1 below shows the percent reduction in P₂O₅ content of thecrystals achieved after the stated retention times in the reactor.

TABLE 1 Time in Seconds % Reduction based on initial P₂O₅ content 2 52.530 60 120 75

This clearly shows that recovery in a one-pass system follows the law ofdiminishing returns, possibly due to the attenuating effect of the verysmall shattered crystals.

In order to improve phosphoric acid recovery, after the first sonicationstage, the suspension can be passed through a high efficiency cyclone inorder to separate the crystals. The oversized crystals can then bereturned for a second pass through the sonicator.

Examination of the treated gypsum under a microscope revealed asignificant increase in the number of very small gypsum crystals and thepresence of hemi-hydrate in addition to di-hydrate. As the crystalsbecome smaller so the gypsum will be more difficult to filter and dry,increasing operating costs. Recovery to be aimed at thus depends to alarge extent on economic considerations.

Depending on the amount water soluble P₂O₅ remaining in the treatedgypsum the latter can be used in the cement industry and/or the gypsumproducts industry. If necessary, the amount of P₂O₅ remaining in thephospho gypsum can be reduced even further. In the case of the cementindustry, for example, levels of water soluble P₂O₅ of greater than0.06% are not acceptable because of increased setting times.

This further treatment consists of the addition of a neutralising agentand further sonication. In this further treatment much of the remainingwater soluble P₂O₅ in the gypsum is converted to an insoluble phosphatewhich does not interfere with the reactions taking place during thesetting of the cement. The water soluble P₂O₅ content is typicallyreduced to less than 0.06% by weight, preferably to about 0.01% byweight.

The phospho gypsum may either be treated directly in the NAP to producewater insoluble calcium phosphate, or after the economically recoverableP₂O₅ has been removed previously.

Suitable neutralising agents have been found to be: Lime (calcium oxideor calcium hydroxide), Limestone (calcium carbonate), Portland cement,ground Portland cement clinker or any other material such as magnesiumcarbonate or dolomite which, on reaction with phosphoric acid, willproduce water insoluble phosphates.

Table 2 below shows details of some of the experiments conducted usingthe treated gypsum of the invention and the resulting cement settingtimes. The latter were determined by an ISO accredited cement laboratoryusing standard ENV methods. The quantity of neutralising agent used inthe experiments was based on the conversion of all P₂O₅ present in thegypsum to water insoluble phosphates.

TABLE 2 Cement Setting Power Depth of Retention Slurry TimesNeutralising Input Reactor Time Solids Initial Final agent KW mm Sec %Min. Min. Lime 2 10 2 30 200 230 Limestone 0.7 2 0.5 30 290 350 2 10 123 215 255 2 10 0.5 26 215 250 2 10 2.4 30 185 280 (1) — — — — 347 420(2) 2 10 1 27 193 290 OPC 2 10 1.5 30 180 215

To demonstrate the-effect of sonication on cement setting times, anexperiment was conducted in which a control slurry of phospho gypsum andlimestone was vigorously agitated for 15 minutes, filtered, rinsed withfresh water and dried. This was interground with cement clinker and thesetting times determined. (1) in the table refers. (2) refers to thesame slurry, which was sonicated immediately after the addition of thelimestone. The setting times were established by drying the filtercakewithin hours after production.

The above results show that the setting time of a cement using phosphogypsum treated in accordance with the invention is significantlyreduced. In addition, it can be seen that cement setting times can beadjusted as required by choice of neutralising agents, reactor depth,power settings, slurry velocities and slurry solids contents. Hence foreach application optimum conditions can be established.

In the pilot plant tests, based on local phosphoric acid plantoperations, the preferred operating conditions of the pilot plant werefound to be:

1. Power input—full;

2. Slurry solids content—26%;

3. Limestone addition—4% based on dry phospho gypsum containing not morethan 1.3% total P₂O₅;

4. Reactor depth—10 mm (which for the 2 kW pilot plant NAP was found tobe the most suitable); and

5. Retention time of slurry in reactor—1.2 seconds.

Sufficient treated gypsum was prepared for the full scale production ofcement in two cement plants.

Setting times reported here were all determined in the same laboratoryas mentioned above.

Plant A Produced 350 ton cement. Initial Set 200 minutes. Final Set 260minutes. Typical setting times using natural gypsum were: Initial Set200 minutes. Final Set 283 minutes. Plant B Produced 135 ton cement.Initial Set 242 minutes. Final Set 294 minutes.

XRD traces of the treated gypsum used showed the presence of limestone,calcium oxide, calcium sulphate di-hydrate and calcium sulphate halfhydrate. This shows that many of the original gypsum crystals had beenshattered and the limestone decomposed.

Thus, depending on local conditions and economic considerationssonication of phospho gypsum can be applied to provide additionalrecovery of P2O₅ in a phosphoric acid plant in addition to rendering ofthe phospho gypsum suitable for use in the gypsum product industry, andas a cement set controller in the cement industry. The process of theinvention is also believed to be environmentally friendly in that itdoes not require high energy inputs. Further, the need to produce costlywaste dumps for untreated phospho gypsum is reduced.

What is claimed is:
 1. A process for recovering phosphoric acid fromphospho gypsum produced as a by-product in a phosphoric acid productionprocess and having water soluble P₂O₅ bound in the crystal latticethereof, the process comprising: a) forming a suspension of phosphogypsum in an aqueous medium; b) subjecting the suspension to ultrasonicwaves under conditions suitable to shatter the phospho gypsum crystallattice releasing the bound water soluble P₂O₅ into the aqueous mediumto produce phosphoric acid and a depleted phospho gypsum; c) separatingthe phosphoric acid from the depleted phospho gypsum and recovering theseparated phosphoric acid; and d) optionally further treating thedepleted phospho gypsum in a step to further reduce the level of anyremaining water soluble P₂O₅ present in the depleted phospho gypsum,wherein the level of the water soluble P₂O₅ in the depleted phosphogypsum is reduced to about 0.01% by weight.
 2. A process according toclaim 1, wherein the further treatment step comprises the sonication ofthe depleted phospho gypsum in an aqueous medium and/or the addition ofa neutralizing agent to the aqueous medium to convert the water solubleP₂O₅ into an insoluble phosphate.
 3. A process according to claim 1,wherein the suspension is subjected to ultrasonic waves having afrequency of less than or equal to 20 kHz.
 4. A process according toclaim 3, wherein the suspension is simultaneously subjected to twodifferent frequencies less than or equal to 20 kHz.
 5. A processaccording to claim 4, wherein the suspension is simultaneously subjectedto a first frequency of about 16 kHz and to a second frequency of about20 kHz.